The present invention relates generally to amplifiers and, more particularly, to power amplifiers used in communication systems, such as in base stations of two-way communications systems. It is well known that power amplifications stages of typical radio frequency (RF) transmitters behave in a nonlinear fashion when operated near peak capacity. Amplifiers have a linear region of operation at low powers. That is to say, at low input and output powers, the power of the output signal is linearly related to the power of the input signal. As an amplifier is driven at increasing power levels, a graph relating its output and input powers exhibits a characteristic “drop-off” and the output power is no longer linearly related to the input power.
A simple solution to this problem is to require that the power amplifier be operated only in the linear region below its saturation point. Backing off a power amplifier in this fashion requires that a larger, higher powered and more expensive device is needed to achieve a given output power. Also, even if an amplifier is backed off to operate in its linear region, there may still be a problem with phase distortion, which may be of concern when modulation schemes that rely in part on phase modulation are used.
Therefore, amplifier designers have devised various techniques to compensate for the distortion that characterizes of operation in the nonlinear region. One technique known in the art uses predistortion of the amplifier input. U.S. Pat. No. 5,867,065 issued to Leyendecker discloses a technique for predistortion in a linear transmitter amplifier used in a pager. Predistortion in the Leyendecker transmitter is performed on the digitally modulated input signals at baseband frequency. Then the predistorted signals are upconverted to an intermediate frequency and used to modulate an RF carrier that is input to the power amplifier. For use in the predistortion process, a portion of the amplifier output is demodulated from the carrier and downconverted to baseband again. This technique of predistortion requires at least one local oscillator, and control circuitry such as a phase locked loop to maintain the upconversion, downconverstion, modulation and demodulation in synchronism. The circuitry needed to support digital predistortion at of the baseband signals is, therefore, relatively complex. Moreover, the specific digital predistortion process is dependent on the type of digital modulation employed.
The goal of the present invention is to provide a much simpler technique for predistortion of a power amplifier, without the need for complex supporting components, such as local oscillators and timing circuitry. A related goal is to provide a predistortion technique that is independent of the specific type of modulation employed.